The present invention relates to a semiconductor device containing a resistor circuit that is made of polycrystalline silicon.
Resistors employed in semiconductor integrated circuits include diffused resistors, which are obtained by implanting in a single crystal silicon semiconductor substrate an impurity that has a conductivity type reverse to that of the semiconductor substrate, and polycrystalline silicon resistors, which are made of polycrystalline silicon with an impurity implanted therein.
A polycrystalline silicon resistor, in particular, is widely used in a semiconductor integrated circuit due to its advantages of having a small leak current reduced by an insulating film surround the circumference, and having a high resistance caused by defects in grain boundaries.
FIG. 2A and FIG. 2B each show a schematic plan view and a sectional view of a conventional polycrystalline silicon resistor circuit.
The polycrystalline silicon resistor is made by implanting a P type or N type impurity in a polycrystalline silicon thin film that is formed through deposition (for example, LVCD) on an insulating film, and then shaping the film into a resistor shape through photolithography.
The impurity implantation is for setting a resistivity to the polycrystalline silicon resistor, and a P type or N type impurity is implanted at a concentration of 1×1017/cm3 to 1×1020/cm3 in accordance with the desired resistivity.
Terminals are formed by disposing a contact hole and a wiring on each end of the resistor to obtain the potential therebetween. In order to obtain sufficient ohmic contact between the polycrystalline silicon layer and the metal wiring at the terminal, impurities are implanted to have a high concentration equal to or larger than 1×1020/cm3.
Polycrystalline silicon resistors are used in resistor groups 201 to 204 shown in FIG. 3 to form a resistor circuit. Each of the polycrystalline silicon resistors is made of polycrystalline silicon 3 composed of a low concentration impurity region 4 and a high concentration impurity region 5, which are formed in an insulating film 2 on a semiconductor substrate 1, and the electric potentials of a terminal 101 to a terminal 105 are obtained from metal wirings 7 via contact holes 6 provided above the high concentration impurity regions 5 as shown in the schematic plan view of FIG. 2A and the schematic sectional view of FIG. 2B.
To obtain various electric potentials from the resistor circuit, Resistor Group One (201) to Resistor Group Four (204) can have various structures in which each resistor as a unit is connected in series or in parallel to one another. To stabilize the resistance of each resistor group a metal cover on the resistor group is formed and connected to a terminal at one end of the resistor group. This is employed for the following two reasons.
The first reason is to stabilize the polycrystalline silicon resistor. Since polycrystalline silicon is a semiconductor, formation of a wiring or an electrode on a polycrystalline silicon resistor leads to a change in the resistance of the resistor caused by depletion or accumulation of charge in polycrystalline silicon depending on the relative relation between the electric potential of the wiring or the electrode and that of the polycrystalline silicon resistor.
Specifically, existence of a wiring or an electrode having higher electric potential than a polycrystalline silicon resistor immediately above a part of the polycrystalline silicon to which P type impurities were implanted, causes depletion of charge in the P type polycrystalline silicon, which increases the resistance. When the electric potential relation is reversed, the resistance decreases due to the occurrence of accumulation.
The resistance can be kept constant by intentionally arranging a wiring above the polycrystalline silicon, the wiring which has an electric potential close to that of polycrystalline silicon to avoid such resistance shifts. The plan view of FIG. 2A shows an example according to this principle in which an electrode connected to one end of the polycrystalline silicon resistor is extended to the resistor to maintain the electric potential constant.
This phenomenon depends not only to the wiring above the polycrystalline silicon but also to a condition below the polycrystalline silicon; the resistance changes depending on relative relation in electric potentials between the polycrystalline silicon resistor and the semiconductor substrate below the polycrystalline silicon resistor. A method has been known, though not illustrated in drawings, for stabilizing the electric potential in this portion by intentionally forming a diffusion region or the like below the polycrystalline silicon resistor in the same manner as the metal wiring described above.
The second reason is to prevent hydrogen, which affects the resistance of the polycrystalline silicon, from diffusing into polycrystalline silicon in a semiconductor manufacturing process.
Polycrystalline silicon is composed of grains having relatively high crystallinity and grain boundaries of low crystallinity, in other words, grains of high trap-level density, which are located between the grains. The resistance of a polycrystalline silicon resistor is determined mostly by trapping of carriers (electrons or holes) at trap-levels which exist in large number in these grain boundaries. However, when hydrogen, which has a large diffusion coefficient, is generated in a semiconductor manufacturing process, the generated hydrogen atoms easily reach polycrystalline silicon and are trapped at trap-levels, thereby changing the resistance. Processes that generate hydrogen include sintering process, which is performed in hydrogen atmosphere after metal electrode is formed, and plasma nitride film forming process, which uses ammonia gas, a composite of nitrogen and hydrogen.
The resistance shifts of polycrystalline silicon due to hydrogen diffusion can be reduced by covering the polycrystalline silicon resistor with a metal wiring.
The method of stabilizing the resistance of polycrystalline silicon is disclosed, for example, in JP 2002-076281 A.
However, the conventional method for stabilizing the resistance of polycrystalline silicon has a problem in that the metal over the polycrystalline silicon is more susceptible to other factors, for example, charging by plasma, heat, and mechanical stress in a semiconductor manufacturing process than hydrogen. The influence of such factors affects the polycrystalline silicon through the metal placed above the polycrystalline silicon, resulting in the resistance shift.